Bakemono, +2

You have a +2 bonus to attack and damage rolls made with this magic weapon.

Critical hits and misses. With this sword, rolling an 18 or 19 counts as a critical hit, but rolling a 17 or 16 counts as a critical miss. Whenever you roll a critical hit, add a charge to the sword. The sword has 8 runes across the blade, and with each charge, a rune lights up with a blue-green color, making the number of charges visible.

Spirit. This sword is possessed by a spirit. Attuning to the sword binds the sword and its spirit to you. You are unwilling to part with the sword, keeping it on your person at all times. While attuned to this weapon, you have disadvantage on attack rolls made with weapons other than this one.

Real Spirit. The spirit is, in fact, an Oni trapped in the sword by angels 200 years ago. The Oni uses its deception to pose as an angel, saying it’s helping the character. It speaks to the character, encouraging the use of the blade, telling them it will do more damage over time. Getting a critical hit does, in fact, raise its damaging potential, but also its power of deception (see charges). Because of the curse, the character is eager to believe the spirit in the sword. The character can roll an Insight check against the Deception score of the Oni, but because of the curse, the character rolls with Disadvantage and the Oni rolls with Advantage.
In fact, the Oni itself also grows in strength, and when the sword is fully charged, the Oni can get out of the sword. It emerges still posing as an angel, deceiving the players as a friend. At some point, the Oni redirects its anger for being trapped in the sword by trying to kill the character. See Oni stat block for information on spell use and attack.

Charges: Each critical hit ads a charge to the sword. Each charge lights up a(n) (additional) rune with a blue greenish color.
Each charge adds +1 bonus to damage rolls (not to attack rolls).
Every two charges also add +1 to the Deception score of the Oni.

When the Oni escapes from the fully charged sword, it then becomes a +2 weapon with no other properties.

The DM can decide to give the information under "Real Spirit" to the character or not.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Damage, Combat, Versatile, Sap